Respiratory assistance devices such as artificial respirators are used in medical practice. A typical respiratory assistance device includes an oxygen supply source such as an oxygen tank, an inspiratory pipe connected to the supply source, a mask attached to a tip of the inspiratory pipe, an expiratory pipe branched from the inspiratory pipe, an expiratory valve fixed to a tip of the expiratory pipe, etc. (for example, Japanese Patent Application Laid-Open Nos. Hei. 02-131765, Hei. 02-131773, Hei. 02-131774, and Hei. 05-245204).
Various methods such as a controlled ventilation (Controlled Ventilation) method used for a patient in the absence of spontaneous breathing (a patient under general anesthesia, during cardiopulmonary resuscitation, or in a critical condition) and an assisted ventilation (Assisted Ventilation) method in which a positive pressure (Positive Pressure) is created in an air passage in synchronization with the spontaneous breathing of a patient are employed for such respiratory assistance devices.
In a respiratory assistance device employing any of these methods, oxygen sent out from the oxygen tank is supplied to lungs as inspiratory air via the inspiratory pipe. The oxygen supplied to the lungs is then exhaled by the lungs as expiratory air. If the expiratory air is discharged into the expiratory pipe, a pressure in the expiratory pipe is increased. A control unit then receives a sensing signal from a pressure sensor having detected the pressure increase in the expiratory pipe and opens the expiratory valve. In this manner, the expiratory air is emitted to the outside from the expiratory pipe.